Pupil size

[Charic]

If my  pupils only dilated to a proven maximum of 4mm (its hard to accurately measure in a dark room with dimmed red torch ?) they would reduce the effective aperture of my 7x50  to the equivalent of using 7x28 binoculars, so only getting roughly half the benefit of owning 7x50!

My 7x50's were purchased some 29 Years ago, and although I really like them for the brighter, wider views they offer, my eyes were so much younger back then,  I feel its now  time,  that better optics, and  Winter-proofing is in order, after reading so much information on the internet.

If I now purchase an 8x40 this would allow my 4mm pupil to reside within the 5mm exit pupil giving a slight leeway, as the pupil is not totally fixed through the  duration of the observing session, unless I imagine, your totally dark adapted, and out in the wilds.

My point is, should I try to get something that is solely matched to a proven maximum eye pupil dilation (If that can be accurately  measured from within the home) which totally limits the binocular for my own personnel use, or  stay with something on the brighter side (larger exit pupil) letting  my eyes automate the effective aperture of the binocular in use, and still be useful if sharing the binoculars.

I could not manage to overcome my New Nikon action (EX) 10x50. It felt like the IPD was too narrow , and  just felt awkward in use, whereas, all my other optics fit comfortably (5 in all?). I also noted that the left eye was darkening? ( was this the IPD setting or the lower 5mm exit pupil) which makes my decision even harder  If I were to  purchase some  8x40's.

Only Argos in my City? has binoculars, so not the best place to try before I buy, and even the major Cities here, Inverness/Aberdeen are very limited in specialist shops to try these binoculars out. And frankly, testing them during the day is no real measure of how they will work in low light conditions. if only more vendors could possibly allow the purchaser the chance to try first, and return if not satisfied, until you find what's best for you. 

The intended use, is solely for Star gazing. My nights are far too bright during the Summer Months to warrant setting up my telescope, so a  wide-field view of the remaining visible Constellations will be just fine for now. 

Any thoughts or advice on this matter?

[ronin]

I would suspect your eyes dilate to bigger then 4mm, you really need someone else to perform the measurement and someone who knows what to do and how to take the measurement.

I say this as if you are convinced it is 4mm you could coinvince them of this also, so they say 4mm to match what you state.

Until this can be done there is little point in conjecture.

Get the binocular that is comfortable and use them, ignore pupil size and exit pupil, as a binocular exit pupil is fixed then you are always either wasting light or not getting enough. If your pupil is 4.9mm then some is wasted, if your pupil is 5.1 then there is aperture for a bit more. So whatever you have is never right.

Way too far for you to get to but Rutland Water have a large birdwatching event on in August and there are something like 2000+ different binoculars there to try, great way to peer through just about every half decent set of binoculars in the UK to find the ones that suit you.

http://www.birdfair.org.uk/exhibitorcats/optics-cameras-accessories/

In the 45 years I have bought binoculars I have never given a single second of thought to exit pupil and my eye, and I bet 99.99% of people don't either. And lets face it it is easy - take the 2 numbers on the front of the binoculars and divide one by the other.

If you suspect a problem with your eyes then an optician is the place to go, a good one with all the equipment to measure and diagnose them. Certainly what I suspect is standing in front of a mirror with a red light and a ruler is not the best optical diagnosis I can think of.

Advice for binoculars: Bog standard 8x42's as used in bird watching and general observing. For larger IPD you may need to search the manufactirers specifications.

[Charic]

ronin.....Rutland's not out of the question this year, I used to live in Edith Weston some Years ago.

My reason for the in-depth look at exit pupils stems from an article I've been reading by Ed Zarenski (2004)  on the CN forum, and other articles.

I agree with what you say to some extent,  How many folk really worry about matching their pupils to exit pupil? but in fairness,  there must be some reason why it should matter, as there'so much written and mis-understood about the subject. 

I like my old 7x50's as I keep harking on about them, but there are  better quality optics out there, and I'd like some water proofing too, so now seems a good time to check things out a little deeper. My 7x50s are bright, but due to my age and pupil size, am I getting the full benefit from the 7x50s or is there a need for a specific Exit size.

If I can study and find good reason, nothing is lost.

My optician doesn't  measure pupil dilation as part of my bi-annual eye check, even asking  seems to draw a blank from them! My IPD is fine on all binoculars except the Nikon Action?

Unless I'm on a road trip soon ( Just been to North Wales! ) there isn't the greatest choice of retailers up here to try some optics, but maybe there's  a Bird fair nearer to this location. The Ornithologists do like there 7x32s and 8x40s for good reason. Thanks for your input.

Edit: Scottish Birdfair May 2014......Oh well!

[Charic]

In the 45 years I have bought binoculars I have never given a single second of thought to exit pupil and my eye,

..........I would'nt start now then, as there's a hive of information on the subject!    I've settled for the  Helios 8x40 WA from flo. 5mm exit pupil, 8° aov

[astronymonkey]

I tend to consider exit pupils when buying eyepieces mainly due to my f4.5 scope giving relatively large exit pupils. What ive never understood though is how an exit pupil larger than your own pupil has the effect of stopping down the aperture. As a projected image I would have though it would have limited the field of view but I dont understand the stopping down issue.

Dont get me wrong, im not saying its not true, only I just dont understand why ?!?!?

[Charic]

.......easy really to understand, NOT!     Same thoughts here, but having had many nights and days studying this anomaly. I found this article which makes an interesting read, and goes someway  in explaining as to why it happens. http://www.cloudynights.com/documents/binoexit.pdf

[astronymonkey]

Thanks charic, ill have a better look later on when I get home but it seems like a good read as ive quickly scanned through it.

Cheers

[BinocularSky]

What ive never understood though is how an exit pupil larger than your own pupil has the effect of stopping down the aperture. As a projected image I would have though it would have limited the field of view but I dont understand the stopping down issue.

Easy enough to demonstrate: Make a sub-aperture mask for your scope. Point your scope at a bright bit of sky (NOT the Sun!) and pop an eyepiece into it. Look at the size of the exit pupil. Now see how it changes as you put the mask on the aperture. Note that light paths are reversible: i.e. stopping down the aperture > reducing the exit pupil, from which we may conclude that reducing the exit pupil is equivalent to stopping down the aperture. Look at a distant object with and without the aperture mask. Note that it gets dimmer (and probably sharper) with the mask, but that FoV is not reduced.

Still not convinced? Use a low-power eyepiece with a decent eye relief.  Make some exit pupil stops (small holes in card). You will need to put them inside the eye relief distance so that your eye will still get to the proper place (you will know when it is there as you will be able to see the eyepiece field stop cutting off the image periphery). Again, not no reduction in FoV, but some reduction in brightness. This is exactly equivalent to stopping down with an eye pupil smaller than the exit pupil.

Still not convinced? Shout, and I'll cobble together a couple of ray diagrams that should do it for you.

[Charic]

astronymonkey.......Your eyes will receive their maximum resolution when their not using all their aperture, the same for camera lenses. If you stop a camera lens down the image gets sharper and depth of field increases. Its said from studies that effective apertures for the human eye is between 1.5 - 2mm in diameter.

We all have varying levels of dilation and matching the optics exit pupil to your own pupil size is very good advice for getting the best from your optics. Too small an exit pupil and you may have to hunt around with head/eye movement to align with the image given by the exit pupil. To big an exit pupil causes a wasting of the light from the exit pupil, and some won't enter your pupil, so effectively you are now reducing the aperture.? Too much exit pupil does allow for easier alignment  and less head/eye movement, but if your exit pupil is 7mm and your eye can only accept 5mm you have reduced the 'effective aperture' of the 'exit pupil'. 

As Steve mentions, you may find more by studying the Laws of Optics, but for now, its suffice to say that an exit pupil  of 7.14mm from a 7x50 binocular is  being "aperture reduced" by someone only able to open their pupil to 5mm. Steve often mentions several types of 50mm binoculars that are not true 50mm apertures, caused by the fitting of internal field stops. These stops help to correct outfield aberrations and effectively give you a sharper image, ie stopping down the camera lens. Hope this helps.

[Alien 13]

If i am reading this correct then is the max perceived brightness of a starfield etc limited to the exit pupil alone and not the scope so a 10" reflector will be the same as 40mm binos with the same exit pupil. 

Alan

[Charic]

Alien 13......Hi, Not sure of your question here. Apparent brightness or perceived brightness is the brightness you see from your observation post. This apparent brightness will depend on the aperture of your telescope or binocular, the health of your eyes and the seeing conditions

Pupils can vary in dilation say from between 2mm to over 7mm. The younger you are, the better.

The larger exit pupil allows more range for your eye pupil to operate  over during various levels of darkness, thus effectively giving you the brightest image, although a large exit pupil and low magnification may not be the best combination for the faintest of Stars, here the increased contrast between dark sky and Star is more afforded by using a  higher magnification, which in-turn reduces the exit pupil size. Your pupil will stop itself down to achieve the best solution. Too much exit pupil cant be accomodated by a smaller eye pupil, therefore, back to  the exit pupil aperture reduction.

Your brain/eye co-ordination will integrate the two incoming signals from your binocular possibly producing a brighter single image compared to your single eyed telescope image.

[Stu]

Just found this which is quite interesting regarding binoculars:

http://www.nikon.com/products/sportoptics/how_to/guide/binoculars/basic/basic_05.htm

Charic, I think Alan's question was more 'If I view through a 10" scope and a pair of 8x40 binos with the same 5mm exit pupil, would the maximum perceived brightness be the same?'

Assuming your pupil was dilated more than 5mm, I assume the answer is yes, but the difference will be in the magnification. In the 10" scope, a 5mm exit pupil is given will x50 magnification. So the scope is able to deliver the same brightness but at much higher power.

[Alien 13]

Just found this which is quite interesting regarding binoculars:

http://www.nikon.com/products/sportoptics/how_to/guide/binoculars/basic/basic_05.htm

Charic, I think Alan's question was more 'If I view through a 10" scope and a pair of 8x40 binos with the same 5mm exit pupil, would the maximum perceived brightness be the same?'

Assuming your pupil was dilated more than 5mm, I assume the answer is yes, but the difference will be in the magnification. In the 10" scope, a 5mm exit pupil is given will x50 magnification. So the scope is able to deliver the same brightness but at much higher power.

Thanks for that and Charic too for the answers it was the thought that for a given exit pupil while the image scale changes the overall percieved brightness might not that i initially couldnt get my head around. I then thought of the analogy with lenses in that an F4 50mm and F4 500mm would expose a sensor at the same rate but differing scales.

Alan

[Stu]

I also found this link which I think is very useful for explaining things, particularly relating to surface brightness:

http://rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

Stu

[Alien 13]

I also found this link which I think is very useful for explaining things, particularly relating to surface brightness:

http://rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

Stu

Thats great its exactly what i was trying to figure out good find.

Alan

[BinocularSky]

I also found this link which I think is very useful for explaining things, particularly relating to surface brightness:

http://rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

Stu

Mostly good, but it misses the transmission factor out of the equations. This is always less than 1, so his " the brightest image you can get, the surface brightness is exactly the same as what you see with your eye" should read " the brightest image you can get, the surface brightness is never as bright as what you see with your eye."

I expand on this to some extent at http://astunit.com/astunit_tutorial.php?topic=telescope

[RikM]

If i am reading this correct then is the max perceived brightness of a starfield etc limited to the exit pupil alone and not the scope so a 10" reflector will be the same as 40mm binos with the same exit pupil. 

Alan

As I understand it, yes, for extended objects but I think the number of stars you will see is down to the aperture alone. I am not sure I understand it though.

[Stu]

As I understand it, yes, for extended objects but I think the number of stars you will see is down to the aperture alone. I am not sure I understand it though.

The limiting magnitude of stars that you see varies with a number of factors. Aperture is of course one of them, but increased magnification also allows you to see fainter stars, plus things like mirror reflectivity etc etc

I think the magnification effect is infact exit pupil related, darkening the sky background so allowing fainter stars to be seen

[Charic]

The brightness of an object is a function of the size of the telescope and its magnification.  The more light you have to begin with ( larger aperture), the more you can magnify before the image becomes too dim. All very simple after a few hours Physics revision?    Its great that these little extra snippets of information seem to appear from no-where, even after my own intensive searches. I'm a firm believer that any subject can be discussed but sometimes we, as individuals,  just cannot grasp the issue or reasoning, or understand fully (at the time) someone's  question or specific needs, then someone comes along and just says one paragraph different, or displays an example and 'Bingo' it all becomes clear. Thats the joy of this forum, everyone is eager to help. Thanks everyone.